In wavelength division multiplexing (WDM), a plurality of optical signals or channels are carried over a single optical fiber with each channel being assigned a particular wavelength. Optical signals or channels in a WDM system are frequently transmitted over optical fibers, which typically can transmit signals over distances of approximately 50-100 km without significant attenuation. For distances beyond 100 km, however, optical amplifiers are required to compensate for optical fiber loss.
Optical amplifiers have been developed which include an optical fiber doped with erbium. The erbium-doped fiber is “pumped” with light at a selected wavelength, e.g., 980 nm, to provide amplification or gain at wavelengths within the low loss window of the optical fiber. However, erbium doped fiber amplifiers (EDFA) do not uniformly amplify light within the spectral region of 1525 to 1580 nm. For example, an optical channel at a wavelength of 1540 nm is typically amplified 4 dB more than an optical channel at a wavelength of 1555 nm. While such a variation in gain can be tolerated for a system with only one optical amplifier, it cannot be tolerated for a system with plurality of optical amplifiers or numerous, narrowly-spaced optical channels. In which case, much of the pump power supplies energy for amplifying light at the high gain wavelengths rather than amplifying the low gain wavelengths. As a result, low gain wavelengths suffer excessive noise accumulation after propagating through several amplifiers.
Accordingly, fixed gain flat filters (GFF) and other optical equalizers have been used to balance the gain of EDFA systems. However, due to GFF's spectrum ripple behavior, the unflatness across channels become much more pronounced after multi-stages of EDFA amplification. Eventually, the ripple behavior makes a part of the channels unacceptable. Accordingly, gain equalization that equalizes all channels without introducing a ripple effect or with minimal ripple effect is needed.
Further, EDFA systems are limited to use primarily with C-band signals (1530 nm to 1565 nm). However, to meet exponentially increasing data transmission demands of big data and cloud storage, utilizing wider signal bandwidth beyond transitional C-band have been proposed. As a practical means of increasing the bandwidth, extending to L-band is currently the most realistic candidate. Generally, Raman-type amplifiers are used with L-band (1565 nm to 1625 nm) signals based on their superior amplification at L-band frequencies. Unfortunately, the GFF and optical equalizers used with EDFA systems generally are not applicable to Raman-type amplification, and if used, result in even more severe rippling problems than occur with C-band signals.